Radiation therapy is currently one of the most useful methods of treating cancerous tumors. However, radiation therapy damages normal tissue surrounding the tumor (U.S. Pat. No. 5,599,712, incorporated by reference herein in its entirety). This damage can include fibrosis, remodeling of the extracellular matrix, vascular damage, aberrant angiogenesis, pneumonitis, atherogenesis, osteonecrosis, mucositis, immunosuppression and functional impairment (U.S. Pat. No. 5,616,561, incorporated by reference herein in its entirety). As a result of these radiation-induced side effects, techniques have been developed to minimize radiation-induced damage to surrounding normal tissues by limiting radiation to the lowest level effective for cancer treatment. Since there is a direct relationship between the amount of radiation and the effectiveness of the treatment, this method compromises the overall effectiveness of the treatment.
For some cancer patients, hematopoietic toxicity frequently limits the opportunity for radiation dose escalation (Watanabe et al., British J. Haematol. 94:619–627 (1996)). Repeated or high dose cycles of radiation therapy may be responsible for severe stem cell depletion leading to important long-term hematopoietic sequelea and marrow exhaustion (Masse et al., Blood 91:441–449 (1998). Such stem cell depletion leads to depletion of the full range of hematopoietic lineage specific cells, including megakaryocytes, platelets, monocytes, neutrophils, and lymphocytes, and the resulting complications of such depletion. For example, in patients suffering from depressed levels of platelets (thrombocytopenia) the inability to form clots is the most immediate and serious consequence, a potentially fatal complication of many therapies for cancer. Such cancer patients are generally treated for this problem with platelet transfusions. Other patients frequently requiring platelet transfusions are those undergoing bone marrow transplantation or patients with aplastic anemia. Platelets for such procedures are obtained by plateletpheresis from normal donors. Like most human blood products, platelets for transfusion have a relatively short shelf-life and also expose the patients to considerable risk of exposure to dangerous viruses, such as the human immunodeficiency virus (HIV).
The administration of hematopoietic growth factors may reduce short-term side effects induced by radiation, but has been hypothesized to cause long-term hematopoietic damage (Masse et al., 1998; Watanabe et al., 1996). Several studies have suggested that co-administration of negative hematopoietic regulators can minimize radiation therapy-induced myelotoxicity by reducing the number of progenitor cells that enter the cell cycle. (Watanabe et al., 1996; Dunlop et al., Blood 79:2221–2225 (1992); Paukovits et al., Blood 81: 1755–1761; Bogden et al., Annals N.Y. Acad. Sci. 628:126–139 (1991); Deeg et al., Ann. Hematol. 74:117–122 (1997); Masse et al., 1998). This treatment is based on the premise that hematopoietic stem cells are relatively protected from radiation-related toxicity when quiescent, particularly when the malignant cells are proliferating (Deeg et al., (1997)).
Bone marrow contains pluripotent stem cells that are capable of reconstituting the entire hematopoietic system. Bone marrow transplantation has been used to treat various intractable hematopoietic diseases including leukemia and severe aplastic anemia. (U.S. Pat. No. 5,186,931, incorporated by reference herein in its entirety.) Typically, a bone marrow transplant patient is subjected to irradiation to reduce the leukocyte count to zero, followed by transplantation of bone marrow cells which function by producing a sufficient number of normal leukocytes. However, various complications, such as death, infectious diseases, graft versus host disease, radiation nephritis, and interstitial pneumonia frequently occur during the time period between transplantation and the return to normal white blood cell levels after transplantation.
As a result of these frequent side effects, no satisfactory methods are currently available for supporting bone marrow transplantation which are capable of both increasing survival of bone marrow transplant patients and also accelerating the reconstitution of the hematopoietic system of the patient.
Chronic radiation injuries, such as radiation nephropathy, have been viewed as inevitable, progressive and untreatable (Moulder et al., Bone Marrow Transplantation 19:729–735 (1997)). The progressive and untreatable nature of late tissue damage follows from the assumption that the injury is due to delayed mitotic cell death resulting from genetic injury that is produced and irrevocably fixed in place at the time of irradiation (Moulder et al., 1997). Under this view, the only way to decrease the probability of injury is by limiting the radiation dose or shielding the at risk organs.
However, recent results indicate that late-onset radiation-induced tissue injury involves complex and dynamic interactions among parenchymal and vascular cells within a particular organ (Moulder et al., 1997). This model of chronic radiation injury suggests that pharmacological intervention after radiation exposure would be effective.
Thus, despite advances in the field of radiation therapy, prior art methods have proven to be of limited utility in minimizing radiation-induced tissue damage, and improving the efficacy of tumor radiation therapy and bone marrow transplantation. Thus, there is a need for improved therapeutic methods to mitigate radiation induced tissue damage and to improve the effectiveness of radiation therapy. Furthermore, the ability to stimulate endogenous platelet formation in thrombocytopenic patients with a concomitant reduction in their dependence on platelet transfusion would be of great benefit. In addition the ability to correct or prevent thrombocytopenia in patients undergoing radiation therapy or chemotherapy for cancer would make such treatments safer and possibly permit increases in the intensity of the therapy thereby yielding greater anti-cancer effects.